TypedArray.prototype.findLast()

The findLast() method of TypedArray instances iterates the typed array in reverse order and returns the value of the first element that satisfies the provided testing function. If no elements satisfy the testing function, undefined is returned. This method has the same algorithm as Array.prototype.findLast().

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Syntax

js

findLast(callbackFn)
findLast(callbackFn, thisArg)

Parameters

callbackFn

A function to execute for each element in the typed array. It should return a truthy value to indicate a matching element has been found, and a falsy value otherwise. The function is called with the following arguments:

element

The current element being processed in the typed array.

index

The index of the current element being processed in the typed array.

array

The typed array findLast() was called upon.

thisArg Optional

A value to use as this when executing callbackFn. See iterative methods.

Return value

The last (highest-index) element in the typed array that satisfies the provided testing function; undefined if no matching element is found.

Description

See Array.prototype.findLast() for more details. This method is not generic and can only be called on typed array instances.

Examples

Find the last prime number in a typed array

The following example returns the value of the last element in the typed array that is a prime number, or undefined if there is no prime number.

js

function isPrime(element) {
  if (element % 2 === 0 || element < 2) {
    return false;
  }
  for (let factor = 3; factor <= Math.sqrt(element); factor += 2) {
    if (element % factor === 0) {
      return false;
    }
  }
  return true;
}

let uint8 = new Uint8Array([4, 6, 8, 12]);
console.log(uint8.findLast(isPrime)); // undefined (no primes in array)
uint8 = new Uint8Array([4, 5, 7, 8, 9, 11, 12]);
console.log(uint8.findLast(isPrime)); // 11

All elements are visited and may be modified by the callback

The following examples show that all elements are visited, and that the value passed to the callback is their value when visited:

js

// Declare array with no elements at indexes 2, 3, and 4
// The missing elements will be initialized to zero.
const uint8 = new Uint8Array([0, 1, , , , 5, 6]);

// Iterate through the elements in reverse order.
// Note that all elements are visited.
uint8.findLast((value, index) => {
  console.log(`Visited index ${index} with value ${value}`);
});

// Shows all indexes, including deleted
uint8.findLast((value, index) => {
  // Modify element 3 on first iteration
  if (index === 6) {
    console.log("Set uint8[3] to 44");
    uint8[3] = 44;
  }
  // Element 3 is still visited but will have a new value.
  console.log(`Visited index ${index} with value ${value}`);
});
// Visited index 6 with value 6
// Visited index 5 with value 5
// Visited index 4 with value 0
// Visited index 3 with value 0
// Visited index 2 with value 0
// Visited index 1 with value 1
// Visited index 0 with value 0
// Set uint8[3] to 44
// Visited index 6 with value 6
// Visited index 5 with value 5
// Visited index 4 with value 0
// Visited index 3 with value 44
// Visited index 2 with value 0
// Visited index 1 with value 1
// Visited index 0 with value 0

Specifications

Specification
ECMAScript Language Specification
# sec-%typedarray%.prototype.findlast

Browser compatibility

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See also